1. Field of the Invention
This invention relates to a fuel cell system.
2. Description of Related Art
An available fuel cell has a structure in which an electrolyte membrane is sandwiched between catalyst layers of both polarities (cathode and anode), and is further sandwiched between gas diffusion layers of both polarities. When this fuel cell is started up below freezing point, there is a risk that the water generated during power generation will freeze, and the catalyst layers will separate from the electrolyte membrane. Therefore, technology has been proposed for controlling the operation of the fuel cell in such a manner that the amount of water flowing into the cathode-side catalyst layer does not exceed an allowable amount when starting up a fuel cell below freezing point (see, for example, Japanese Patent Application Publication No. 2011-113774 (JP 2011-113774 A)).
In an available fuel cell system, reaction gas is humidified before being supplied to the fuel cell, in order to maintain a good water-containing state in the electrolyte membrane of the fuel cell. On the other hand, in response to demands for simplification of the fuel cell system, the removal of the humidifier which humidifies the reaction gas has been investigated. Since there is a risk of the electrolyte membrane becoming dry during high-temperature operation, when the reaction gas is not humidified, then it is desirable to make the thickness of the electrolyte membrane smaller than in the related art, in order to maintain a satisfactory water-containing state of the electrolyte membrane. This is because it is considered that, when the electrolyte membrane is made thin, a large amount of water moves from the cathode side toward the anode side (this water is called “reversely diffused water” below), thereby preventing the drying of the electrolyte membrane. However, when there is a large amount of reversely diffused water, there is a risk of the catalyst layer separating from the electrolyte membrane due to freezing of the water on the anode side, when the cell is started up below freezing point.
Therefore, technology for adjusting the amount of water flowing into the anode-side catalyst layer is required. This technology is demanded in order to achieve appropriate operation of the fuel cell, and not only for start-up below freezing point. Apart from this, there have been demands for reduced cost, reduced resource usage, simplified manufacturing, and improved performance, and the like, in available fuel cell systems.